The following invention relates to a tread for a tire more specifically for a tread designed for road usage and off-road usage for passenger or light truck vehicles.
Tires generally have a tread that is well-suited for a specific vehicle application. For example, pneumatic tires used primarily on paved surfaces generally have a high proportion of the tread in contact with the road. These tires generally have a net-to-gross ratio in excess of 70%. This means that 70% or more of the area within the tires"" footprint or contact patch is touching the road while 30% less is the void area representing the tires"" grooves within the footprint area. These tires, due in part to the large amount of tread in contact with the road, can yield very good tread mileage sometimes in excess of 75,000 miles during the tires"" normal life.
All season type tires can fall into this category of tires. All season tires have somewhat laterally extending grooves in each shoulder of the tread. These lateral grooves enhance snow traction performance.
If, however, the vehicle owner wants increased traction on snow, tires having a more aggressive open tread pattern have been used for this purpose. These snow tires generally have a lower net-to-gross ratio and almost always a large amount of leading edges on block elements to provide increased traction.
When the vehicle owner wants a tire that can operate both on the roadways as well as in snow, mud and sand, he generally must select an on/off road tread.
Typically, a serious off-road enthusiast may choose a tire designated as a mud tire. These tires are very open, tending not to clog in the mud.
In U.S. Pat. No. 5,085,259, a tread pattern having a plurality of ground engaging block elements arranged in a directional tread pattern is disclosed. This patented tread pattern is employed on a commercially available tire sold as the Goodyear WRANGLER MT. The MT, mud tire designation has signified to off-road enthusiast that the tire was well-suited for off-road applications. A measure of the tires"" off-road worthiness was one of its first applications, the U.S. Army Hummer vehicle. This tread had excellent on/off road capabilities and throughout this patent application several references to testing will be made wherein the comparison or control tire is this prior art tire. As mentioned, the tread pattern of the WRANGLER MT was directional; that means that the tread pattern had a preferred direction of travel. Traction capabilities of the tire are enhanced in the preferred direction of travel. The most easily understood comparison is farm drive tires where the tread lugs pull the tractor through the soil. This nondirectional feature means that the tire must have two sets of moldsxe2x80x94one for each side of the vehicle. This increases tire costs.
In European Patent Application 0 273 056, Noma et al discloses a Radial Tire and Assembly of Radial Tire and Rim that has a tread which is wider (in the axial direction) than the maximum width of said carcass and this document is considered to represent the most relevant state of the art.
Another problem associated with the very aggressive treads is that the high void areas means that when the tire is used on paved roads, the tread is very noisy, exhibits bad vibrational characteristics and has generally poor treadwear.
It is generally understood in the tire art that tread design is a matter of trade-offs aggressive off-road treads have to give up noise performance vibration and treadwear whereas good road tires are poor in off-road applications.
One of the objectives of the present invention is to provide an on/off road tread that has good on-road treadwear while showing excellent off-road traction.
Another objective is to design a non-directional tread with performance characteristics equal to or superior to the directional tread patterned tires.
Another objective of the invention is to design a tread that has a total tread width for off-road including a narrower road contacting central portion for on-road use.
These objective and other advantage can be achieved by the inventive on/off tread for a tire as disclosed herein.
An on/off road tread 40 for a passenger or light truck tire 10 extending circumferentially about the axis of rotation R of a tire is disclosed. The tire 10 has a casing 12, a maximum section width Sw and a maximum section height SH. The tread 40 has a plurality of block elements 41, 42, 43 projecting outwardly from the tire casing 12 from a location at or above a radial location of the maximum section width Sw of the tire 10, but less than 65% of the section height Sw of the tire 10; a central portion 100 of the tread 40 being for on and off-road traction while the remainder of the tread 40 being for off-road traction.
The tread""s central portion 100 is measured between first lateral edge 44 and second lateral edge 46. The distance halfway between the lateral edges 44, 46 defines the equatorial plane (EP) of the tire 10 which divides the tread into a first tread half 110 and a second tread half 120.
In the preferred embodiment of the invention within the central portion 100, the tread 40 has four rows 1, 2, 3, 4 of block elements 41, 42, 43 separated by three circumferentially continuous grooves 54, 55, 56. The first tread half 110 has a first and second row. The first row 1 of shoulder block elements have an array of long block elements 41 extending from the first lateral edge 44 and circumferentially adjacent each long block element 41 is a short block element 43 offset axially award from the first lateral edge 44. Each long and short block element 41, 43 has an axially aligned inner portions 74 divided by an inclined semi-blind groove 76, the semi-blind groove 76 extending less than halfway into each block element 41, 43.
A second central row 2 of block elements 42 are axially inward of the first row. The second row 2 has block elements 42 having an elongated circumferential shape extending inclined circumferentially from the equatorial plane toward an axially outer portion 77, the axially outer portion 77 is divided by an inclined semi-blind groove 78 circumferentially aligned but oppositely inclined relative to the semi-blind groove. 76 of the adjacent shoulder block element 41, 43.
The second tread half 120 has a third row 3 and a fourth row 4, the third row 3 is the same as the second row 2, the fourth row being the same as the first row. Both the third row 3 and fourth row 4 being oppositely oriented relative to the respective first and second rows.
The on/off-road tread for a tire has the central portion of the tread divided into three tread zones 60, 61, 62xe2x80x94a first shoulder zone 61, a second shoulder zone 62 and central zone 60. Each shoulder zone 61, 62 extends from a lateral edge 44, 46 to a circumferential centerline of a circumferential groove 54, 55. The circumferential centerline is defined as midway between peak amplitudes on each of the respective circumferential grove 54, 55, 56. The first and second shoulder zones 61, 62 preferably have equal net-contact areas while the central zone 60 has a net contact area greater than the first or second zones contact area, preferably at least 10% greater than a shoulder zone, more preferably the central zone 60, has a net-to-gross ratio greater than 50%.
The increased traction of the tread is achieved primarily from a plurality of block elements 41, 42, 43 that have a plurality of leading traction edges 50. The sum of the lengths of the leading traction edges 50 adjacent a groove of the tread 40 being at least 150% of the sum of the length of the leading traction edges 50 adjacent a groove of the central portion 100 of the tread 40.